Induction motors supply a back EMF voltage when they are running. During start-up, with no back EMF, the input impedance of the motors is relatively low and the surge voltage is high. This causes unnecessary currents to be drawn from the supply (line). Not only does this require that the motor be designed to withstand these lines, the line must have these large currents available.
A number of solutions have been proposed and are in use for providing a lower voltage during start-up and increasing the voltage as the speed of the motor increases.
One method is the “star-delta” configuration in which the motor is fed from a three phase transformer and the windings of the motor are switched from a star connection to the power line to a delta connection as the motor speeds up. This provides two levels of voltage for starting the motor. This method suffers from the following disadvantages:    1. There are only two voltage levels and there are commutation effects which cause spikes of voltage during changeover from one configuration to the other.    2. There is a need for 6 wires between the power controller and the motor.    3. The motor line current is identical to the current on the supply line, even during start-up.    4. The contactors switching between the 2 modes carry all the motor current.    5. The need for auxiliary large resistors to allow continuous current flow during the switch.
A second method utilizes a tapped autotransformer to vary the voltage. In this method, the voltage to the motor is supplied via a step-down autotransformer having multiple taps. The motor is first connected to the lowest tap and, as the motor speeds up, the input of the motor is transferred to successively higher voltages by changing the tap supplying the voltage. This method suffers from a number of different deficiencies. One is the requirement to switch the entire power being utilized, each time the voltage is changed. A second deficiency is that the coil and core of the transformer must be designed to carry the starting current of the motor. This makes the transformer very large and expensive, with sizes similar to those of the motor itself being common. A third deficiency is that there are serious commutation problems, since the output is disconnected each time the tap is changed. For this reason this methodology is not used extensively. Fourth, the contactors must carry the entire current when the voltages are switched.
A third method uses phase control to vary the voltage. In this method thyristors are used to control the voltage and the phase of firing of the thyristors is used to vary the voltage delivered to an output. This method does not deliver a sinusoidal voltage and its inefficiencies in starting motors are well known. In particular, there is an intrinsic phase delay, especially during start-up and power robbing transients when the thyristor fires. Furthermore, it is generally not possible to use capacitors for improvement of power factor with phase control.
Another issue that arises with control of induction motors is that they are most efficient at full load. When the load is reduced, the core losses remain high and the efficiency drops. It is known that reducing the voltage on induction motors when the load is less than the rated value results in more efficient operation. However, no practical way of implementing such change is known.
Israel patent 133307 filed Dec. 5, 1999, the disclosure of which is incorporated here by reference, describes a system for lighting control in which a transformer primary is placed across the input (between “line” and “return” connections) and the secondary is in series between the load and the line. The secondary is wound and attached to oppose (and thus reduce) the line voltage supplied to the load. This provides a reduced voltage at the load. When the full voltage is needed, the transformer input is disconnected from the return and short circuited, forcing the voltage on the secondary to zero. The secondary can then be short circuited. Multiple transformer stages can be supplied to provide a greater variation in load voltages. For three phase, this configuration is repeated three times.